Conventional alarm systems, such as passive infrared (PIR) intrusion detectors, see extensive use for detecting and deterring intrusions such as break-ins or trespasses. One such alarm system is shown in block diagram form in FIG. 1. Here, an alarm system 10 has a sensing element 20 for detecting intrusions. The sensing element 20 is connected to an alarm indicator 30, typically by a pair of wires 40. The alarm system 10 also commonly has a camera 50 connected to a monitor 60, typically by a coaxial cable 70. In operation, the sensing element 20 and camera 50 are placed in proximity to the location at which intrusions are to be detected/prevented. The sensing element 20 detects intrusions upon this location, while the camera 50 captures images of the location to provide a visual record of any such intrusion. Upon detecting an intrusion, the sensing element 20 transmits a signal to the alarm indicator 30 across the wire 40, indicating that an intrusion has occurred. The alarm indicator 30 then initiates an alarm, such as an audible or visual signal that alerts people to the intrusion. The camera 50 provides positive confirmation if the sensing element did not trigger a false alarm. Thus, the image captured by the camera 50 is transmitted across the coaxial cable 70 and displayed on the monitor 60. In this conventional system, the alarm indicator 30 alerts people, spurring them to activate the camera 50 and/or watch the monitor 60 to determine more information about the intrusion. The use of the camera 50 reduces incidents of false alarm.
While often effective in detecting and monitoring intrusions, conventional alarm systems 10 suffer from certain drawbacks. Often, the sensing element 20 and camera 50 are located at or near the area to be monitored, while the alarm indicator 30 and monitor 60 are placed at a remote area. Accordingly, alarm systems such as alarm system 10 require two transmission lines, i.e. wires 40 and coaxial cable 70, to be extended between these areas. The use of two such cables instead of one consumes extra space and adds cost. Also, as these two areas can be far apart, additional effort and expense are required to install two lines instead of one. In addition, where a sensing element 20 with the associated alarm indicator 30 is already installed, retrofitting such a site would be expensive.
In the prior art, it is also known to use a plurality of cameras connected via a single co-axial cable to a plurality of monitors. Thus the cameras share their signals on the single co-axial cable. However, none of the cameras share their signals with the output of a sensing element which does not output a video frequency signal.
Accordingly, it is desirable to provide an alarm system that employs a single transmission line capable of carrying signals from both sensing element 20 and monitoring devices such as camera 70. The use of a single transmission line eliminates the need for a second line, reducing overall system cost and effort in installation, and permits greater number of existing alarm sensors 20 to be retrofitted with cameras 70 to reduce false alarms.